Many generators, such as those used by electric utilities, operate at almost constant speed. However, some generators are driven by variable speed sources, such as an engine of an aircraft or a flywheel energy storage device. The rotational speed of these variable speed sources can vary significantly during operation. Output voltage of a generator may be proportional to the rotational speed of a rotor of the generator. When the rotational speed of the rotor is too low, the output voltage of the generator may be too low to provide sufficient power to a load. Complicated gear assemblies may be used to interface between a variable speed source and the generator in order to keep the rotational speed of the rotor high enough to generate usable power. However, these gear assemblies may be expensive, complex, and heavy. Heavy gear assemblies may be of particular concern when the generator is part of a portable system or aircraft, where weight of the system can be an important design consideration.